Isolation of A1 and A2 adenosine-receptors from rat brain membranes has been continued. It was found that these receptors could be separated from each other by hydroxylapatite chromatography. It was also found that another adenosine binding protein which showed similar but distinguished properties from A2 receptors was present in the digitonin-solubilized rat brain membranes and could be separated from adenosine receptors by hydroxylchromatography. Moreover, a new affinity chromatography system for the purification of A1 receptor was developed and A1 receptor of the crude solubilized membrane preparation was purified to ~ 50-fold by this method.